Remove namespaced extlib.puppet_config fact

Early versions of Facter 4 supported creating structured facts by
aggregating multiple dotted facts.

ie extlib.puppet_config would become the puppet_config key of a top
level structured fact extlib.

This behaviour caused too many issues with the existing ecosystem so was
removed in https://github.com/puppetlabs/facter/pull/2339

namespacing for facts might still return.

Quoting @gimmyxd on slack...

currently there is no way to switch the auto conversion per fact.  We
are still working on finding the best mechanism to allow enabling dotted
structured facts and ensure a smooth transition from Facter 3 to this
new behaviour: https://tickets.puppetlabs.com/browse/FACT-3000